This invention relates to an analog-type fire detector which detects, as an analog amount, a change in physical phenomena caused by a fire.
In a conventional fire detector, for example in a photoelectric fire detector, to reduce current consumption, a light emitting device is intermittently driven with a period of for example 2 sec, a change of the light from the light emitting device caused by incoming smoke is detected by a photodetector, the photodetection signal is compared with a predetermined threshold value within the light emission drive period, and a switching device is operated when the photodetection signal exceeds the threshold value to lower the impedance between power/signal lines derived from a central signal station to short-circuit therebetween so as to allow an alarming current to be transmitted to the central signal station.
However, it is difficult in the conventional fire detector of this type to accomplish both earlier finding of a fire and prevention of mis-alarming due to its fire detection system by a fixed threshold value. It is also difficult to grasp the status of a fire. In this connection, it has recently been proposed to detect, in the form of an analog amount, a change of smoke density caused by a fire to transmit it to a central signal station so that the signal station can make fire determination based on the analog data.
In such an analog-type fire alarm system, to reduce current consumption, smoke density is usually detected intermittently and the detection operation time is usually as short as about 0.2 msec. Therefore, if the detection output is transmitted to the central signal station as it is, the central signal station cannot surely receive the signal. To solve this problem, it is possible to extend the detection operation time of the fire detector longer than the time for which the central signal station can receive the detection output. In this case, however, an essential object of reducing current consumption cannot be attained. In addition, there has been such a problem that a noise is possibly mingled with data of early detection stage which prevents accurate detection of a fire, causing possible misoperation.
More specifically, in a conventional photoelectric analog-type smoke detector which optically detects smoke density due to a fire and outputs an analog detection signal corresponding to the density of smoke, a pulse duration converting circuit for converting the detection signal into a pulse signal of a pulse duration corresponding to the signal level so as to transmit the analog detection signal in the form of digital data to the central signal station.
This pulse duration converting circuit is generally formed in such a manner that the detection signal and a triangular-wave signal of a predetermined frequency are input to a comparator to obtain a pulse signal having a pulse duration corresponding to the detection signal level by changing the threshold level to the triangular-wave signal by the detection signal.
However, such a conventional pulse duration converting circuit needs a triangular-wave oscillation circuit for generating the triangular-wave signal which is used as a reference for the pulse duration conversion and the circuit arrangement thereof is very complicated, raising the cost thereof.